Oil rotary mechanical pumps are generally employed to produce low vacuum conditions in a pressure range from atmospheric pressure to about 10−1 Pa.
Conventional mechanical pumps comprise a housing having a suction and a discharge port. A stator is located inside the housing and defines a cylindrical chamber where an eccentrically arranged circular rotor equipped with spring-loaded radial vanes. The pumps are immersed in an oil bath to refrigerate and lubricate the pumps and to isolate them from the external environment.
These pumps require a high torque for cold acceleration, because of the low oil temperature. They can give rise to a strain of the electronic control units supplying the motor with power and of the motor itself.
Vacuum pumping devices are also known comprising electronic control devices capable of supplying a variable frequency output signal for driving the pump motor.
These systems are particularly common in high vacuum field and they act on the excitation frequency of the voltage signal by which the electric motor rotating the vacuum pump is powered, causing a variation in the rotation speed of the motor and consequently in the pumping speed of the pump.
Generally, in high vacuum pumping devices, it is advantageous to vary the excitation frequency, in the vacuum pump starting phase, in particular during the acceleration ramp, to limit the acceleration torque.
In the starting phase, the vacuum pump generally sucks air at atmospheric pressure at a much higher pressure than the suction pressure in steady state conditions. Consequently, the acceleration torque is very high and compels to use an electric motor with higher performance than that required by the steady state conditions of the pump.
Using control devices with a variable frequency output signal allows starting the pump at a lower speed, thereby reducing the acceleration torque, and consequently allows using electric motors with performance suitable for steady state conditions of the vacuum pump, such motors are less expensive and cumbersome.
Electronic control devices of that kind are disclosed in documents U.S. Pat. No. 4,664,601 and U.S. Pat. No. 5,110,264.
Thus, it is an object of the present invention to provide a mechanical pumping device for high pressures, which is inexpensive and has a reduced size, while meeting however the aforementioned requirements of high reliability and useful life duration.
It is another object of the invention to provide a mechanical pumping device capable of attaining higher vacuum degrees than conventional systems, while maintaining reduced sizes and costs.
The above and other objects are achieved by a vacuum pumping device according to the invention, as claimed in the appended claims.